1. The Field of the Invention
The present invention relates generally to deep-sea production equipment and in particular to the operation of deep-sea offshore development platforms of the type which are used for conducting various oil field operations in deep water areas. More particularly, the present invention relates to a management system to control rigid risers and tendons in offshore floating platforms in such manner as to maximize fatigue life of risers and tendons and to maintain the spacing between the risers and tendons, thereby allowing smaller offshore vessels to be constructed and reduction in the cost of such operations. The system includes a combination of hardware and software providing data acquisition and analysis to advise the vessel crew members what adjustments are necessary and when they will be necessary.
2. Description of the Prior Art
The search for offshore deposits of crude oil and natural gas is continually being extended into deeper and deeper waters beyond the continental shelf. Where possible, one of the preferred techniques of performing the operations necessary for the production of hydrocarbons from off-shore reservoirs is to erect a structure or operating platform which is, in some fashion, secured to the sea floor. Such a technique may comprise any of a variety of structures including jackup rigs, tension-leg platforms, free-standing or guyed towers. A notable advantage of such structures is their rigid nature which significantly simplifies subsea operations during conditions which exert lateral or vertical forces on the structure. The rigid character of such structures limits their movement to less than 4.degree. of freedom (0.degree. for a rigid tower and up to 3.degree. for a tension-leg platform). It has therefore been found possible to operate two or more production operations from such rigid bottom founded structures. With such rigid structures, operation of two or more conductor pipes simultaneously has not caused significant problems to arise due to the active guidance available with these systems. U.S. Pat. Nos. 2,973,046 and 4,170,266 are illustrative of platforms which are supported in a rigid or semi-rigid fashion from the sea floor. There is, however, a limit to the water depths in which rigid, bottom founded production platforms can be effectively, safely and economically operated. Where the sea depth exceeds this limit, floating platforms, such as ships or semi-submersible platforms, have found application. According to conventional procedures, a floating production vessel is dynamically moored above a well site on the ocean floor. Dynamic mooring, as opposed to rigid bottom founded support, permits the floating platform to dynamically move with up to 6.degree. of freedom under prevailing forces, such as wave action, tidal action, sea currents and wind conditions.
As can easily be imagined, the marine risers required for production operations in very deep water become quite heavy and unwieldy. Unfortunately, the movement of a floating production vessel under the influence of weather, tide and current conditions greatly increases the difficulty of managing the riser as contrasted to the situation of rigid bottom-founded platforms since movement of the vessel excites dynamic motions in the riser systems.
Production vessels, and other apparatus employed in the production of oil offshore, are generally large and very expensive. Their operation involves rates exceeding many thousands of dollars a day, a cost which constitutes a major portion of the overall well and production costs. Thus, it is very important that the operations of each vessel be performed in such a manner as to get the maximum use out of the vessel. Ideally, the situation would be to have a plurality of wells operated from a single production platform thereby obviating the need for redundant vessels. This arrangement causes a problem of multiple risers and the need to keep them appropriately spaced so that they will not be brought into contact by the vessel movement and thereby be subject to damage and/or rupture. Clearly, if certain limits of tension or deflection angle are exceeded, a marine riser can be damaged. Damage may also occur if two risers forcibly come into uncontrolled contact with one another or if equipment being lowered by one riser were to collide with another riser. Production risers, while quite stiff over short distances, are quite flexible over the extended distances which they must traverse in deep water offshore environments. Not only are these risers subject to sea currents (often of different magnitudes and directions at different depths and times), but they are also subject to a condition in which the lower end is pinned to the ocean floor at a stationery spot while the upper end must follow the motions of the floating platforms.
Numerous attempts have been made in the past to deal with problems which arise in the design and management of marine risers for deep offshore oil well production. For example U.S. Pat. Nos. 3,983,706 and 3,817,325 disclose means for providing lateral-support and guidance to marine risers in order to limit their lateral deflection due to currents or platform movement. U.S. Pat. Nos. 3,601,187; 4,576,516 and 4,188,156 describe flexible joints and flexible riser sections for the purpose of accommodating unavoidable deflection of such risers. U.S. Pat. Nos. 3,133,345 and 4,351,261 disclose apparatus and techniques for preventing the violent collision of a riser with the floating platform if the riser were to be separated from the wellhead equipment in a planned or emergency disconnect situation. U.S. Pat. Nos. 3,434,550 and 3,999,617 are directed to methods and apparatus for lightening the riser-mud combination to reduce the compressive and tensional forces placed on the riser. U.S. Pat. Nos. 4,142,584 and 4,198,179 show the conventional approach of ganging multiple risers together as a means of avoiding riser-riser interference.
Riser and mooring management systems have been used in other capacities in the past. In offshore drilling business, mooring management systems are used to insure that the vessel remains over the well being drilled to keep the riser and drill string as vertical as possible. A riser management system has been proposed for managing side by side drilling risers in a vessel equipped with two derricks for simultaneous drilling. An example of this may be found in U.S. Pat. No. 4,819,730. The present riser and tendon management system extends the application of previous systems from the management of the offsets of two drilling risers to the management of offsets and stresses of many (typically 8 to 32) production risers and to the management of offsets and stresses of tendon mooring systems. Also, the intent of previous systems has been to improve operating efficiency. The intent of the present riser and tendon management system is to allow designers to decrease the cost of the floating production systems by decreasing riser spacing and therefore vessel size and by decreasing the amount of steel in the risers or tendons by controlling peak stresses and maximizing fatigue life.